1. Field of the Invention
The present invention relates to a process for forming a coating on a previously pickled metal surface, especially on a metal sheet, and the product obtained by this process, the coating consisting of copolymers based on polydimethylsiloxane imparting anticorrosion and lubricating properties to the surface.
2. Discussion of the Background
The problem of protection of metal surfaces and more particularly of metal sheets against atmospheric corrosion is a continual preoccupation in industry.
In fact, after the operations of hot rolling of flat products and of cold rolling with a view to reducing their thickness, the metal sheets obtained are subjected to a pickling operation and are then stored in a depot or transported to a customer. The metal sheets are then directly subjected to the effects of the environment and corrode as time passes before reaching the user.
It is accepted that atmospheric corrosion represents by far the most important corrosion factor. It acts through the intermediacy of a very thin film of water condensed on the surface of the metal sheets, this film containing chemical species in solution which originate from the atmosphere. The corrosion which arises in this film is of the electrochemical type.
In order to avoid this type of "electrochemical" attack the sheet metal must be isolated from the environment by being coated with a material which is impervious and resistant to this medium such as, for example, by means of oiling. It is also appropriate to endow the surfaces of the metal sheets with a potential for chemical reactivities contrary to those which cause the corrosion.
At present, some products intended for a delayed use are delivered to the customers oiled, the oiling of the products taking place only at the end of the production line, immediately after the pickling. In this case, as soon as the products are received by the customers, the latter must degrease their surfaces before envisaging any use whatsoever.
It is therefore understandable that it is highly advantageous to protect metal sheets temporarily against corrosion for a minimum period of two months while getting rid of the oiling which results in an additional operation by the customer. This period corresponds to the "dead time" between the finishing of the product and its being received by the customer.
Among the known methods for protecting unoiled pickled products can be mentioned corrosion inhibitors, for example amine-based ones, coatings based on a plastic, such as resins or elastomers, and metallic or inorganic coatings.
The major disadvantage linked with the use of conventional anticorrosion products lies in the fact that they exhibit a corrosion resistance which is very limited in time, namely about twenty days. After that, they are no longer effective, the corrosion phenomenon appears and deteriorates the metal sheets.
In another technical field, which is that of the forming of metals by cold plastic deformation, especially by stamping of metal sheets, it is known that the contribution of the frictional forces between a metal sheet and a tool during the deformation represents from 20 to 40% of the total energy absorbed by the operation and is converted entirely into heat energy.
Furthermore, since the deformations applied to the metal sheet are produced under very high pressures during very short times, frictions appear between the metal sheet and the tool during the operation, thus giving rise to adhesions or seizures, that is to say tearing away of metal particles. In extreme cases the sheet metal reaches the limit with mechanical rupture.
Attempts are consequently made to reduce the metal sheet-tool friction coefficient by employing a solid or liquid lubricant which is placed on the sheet metal and/or on the tool and which forms an antifriction layer. It prevents direct contact between the sheet metal to be deformed and the tool and thus reduces the friction.
There are known surface treatments for sheet metal which make it possible to improve the friction coefficient and thus to contribute to a better cold deformation.
Phosphating, in particular, is one of these treatments and the use of zinc phosphates is known to improve the quality of cold deformations. However, phosphating is a chemical conversion process, that is to say that it impairs the chemical properties of the surface of the sheet metal. Now, when the deformation of the sheet metal has been completed, it is possible, depending on the intended industrial application, to apply a chemical treatment to the sheet metal and, when this takes place, reaction incompatibilities may arise and may consequently be detrimental to the performance of the said treatment. In addition, when phosphated metal sheets are to be welded, problems appear as a result of the peculiar behaviour of the sheet metal towards welding products.
There are also known products which are deposited onto a metal sheet with a view to improving the cold deformability of the said sheet, in particular to improve its stamping ductility. In particular, Patent Application FR-88/05334 describes alkali metal salts such as, for example, potassium phosphate, K3PO4, which has lubricating properties when mixed with a lubricating oil at the time of the stamping.